Geological and Sedimentological Controls on Plant Distribution in the Al-Muthana Desert, Southern Iraq

The arid climate of the Al-Muthana Desert, which forms part of the southern Iraqi desert, controls the intensity and spatial distribution of morphoclimatic processes. Drought and desertification associated with global warming have adversely affected the region. The climate is characterized by very low and erratic precipitation, high temperatures, high potential evaporation, and persistent strong winds, which collectively enhance physical weathering and aeolian geomorphic processes. These climatic conditions, together with the geological and geomorphological setting, have resulted in the development of a dynamic desert environment. Although rainfall is infrequent, episodic high-intensity storms can occur. These storms may generate severe flash floods because of the area's lithology, slopes, drainage-network morphology, and surface-sediment properties [1, 2]. Also, desert environments are generally considered less susceptible to biological invasions because of their dry and harsh conditions [3].

Moreover, morphoclimatic processes develop under the influence of various natural factors, including geological structure, topographic configuration, climatic variability, sedimentological characteristics, and the availability and movement of surface water and groundwater. Interactions among geological-geotechnical and hydrological-hydraulic parameters and vegetation are critical for assessing landscape evolution and environmental vulnerability [4].

Existing studies of the Al-Muthana Desert have addressed several aspects of its environmental and geomorphological framework, highlighting the importance of this area as representative of the arid regions of southern Iraq. Previous research has focused on the geographical characterization of natural features, the influence of morphotectonics on topographic evolution, and the relationships among geodiversity, biodiversity, and selected landform units, such as the Al-Salman Depression. Climatic investigations have also assessed differences between bare desert surfaces and vegetated areas, including their implications for ecological functions and land-surface processes. Although these studies have made valuable contributions, detailed understanding of the interactions among climate, geology, geomorphology, and ecology remains limited [5].

Multidisciplinary studies are therefore required to improve assessments of environmental change and landscape evolution in this arid region [6].

Iraq's marked geographical, climatic, and ecological variability makes it an important center of plant diversity in the Middle East. Its diverse landscapes, including mountain ecosystems, steppes, riverine plains, wetlands, and extensive deserts, encompass strong environmental gradients that promote species richness and habitat differentiation. Plant life-history processes, such as seedling establishment, growth, and survival, are strongly controlled by climatic factors. Consequently, plant communities vary across both temporal and spatial scales. Iraq's environmental complexity likely supports a rich wild flora with diverse distributions and adaptations. The country is therefore an ecologically significant reservoir of biodiversity. In this context, the arid and semi-arid regions of Southwest Asia have received particular attention [7, 8].

The Al-Muthana Desert exhibits considerable sedimentological diversity. Its surface ranges from loose aeolian sands and gravels to fine, clay-rich sediments. These different substrates affect hydrological processes, particularly water infiltration, storage capacity, and soil-moisture retention [9]. Consequently, the spatiotemporal variation in sediment moisture is closely related to plant-community composition and vegetation distribution. Soil-water availability is determined by the rainfall regime, including precipitation amount, intensity, frequency, and seasonal timing. These factors affect the germination, establishment, and survival of desert plants. Accordingly, the interaction between sedimentological characteristics and rainfall processes is a major ecological driver regulating vegetation patterns, species composition, and ecosystem functioning in the Al-Muthana Desert [10-12].

Previous studies of the Al-Muthana Desert have examined its geology, geomorphology, climate, and ecology; however, the relationships among geological and sedimentary units, plant-diversity patterns, and community composition remain poorly understood. Research on floristic composition, diversity indices, and vegetation distribution has rarely been integrated with substrate variability. Therefore, the present study evaluates the effects of geological and sedimentological factors on vegetation structure and biodiversity patterns across the Al-Muthana Desert using a multidisciplinary ecological and geological approach.

The study area is located in the Al-Muthana Desert in the southern part of the Iraqi Western Plateau, between 29°45′-31°30′ N and 44°00′-46°30′ E. It covers approximately 44,914 km² (86.9% of the governorate's area). Geological evidence indicates that the area is dominated by Tertiary sedimentary successions, particularly the Umm Er Radhuma, Dammam, Ghar, Euphrates, Nfayil, and Dibdibba formations. These successions consist mainly of dolomitic limestone, chalky dolomite, anhydrite, chert, and interbedded clay horizons containing fossiliferous beds and flint. The formations are overlain by widespread Pleistocene and Holocene deposits, including alluvial-plain sediments, inland sabkha deposits, depression-fill accumulations, and aeolian sands composed mainly of loose gravel, sand, silt, and clay. The study area forms part of the Najd-Hejaz and Western Plateau physiographic units. The terrain is generally flat to gently undulating, with elevations ranging from approximately 10 to 435 m above sea level and a regional slope from south to north. The major wadis-Kur Al-Tair, Shuyaia, Abu Khudair, Al-Kuseer, Sadeer, Abu Ghar, and Al-Athir-shape the geomorphology by influencing sediment transport, surface drainage, and landscape evolution through surface-water and groundwater flow.

The sediments of the Al-Muthana Desert are generally shallow, stony, and weakly developed, with low organic-matter content and abundant gypsum (CaSO₄), reflecting intense evaporation under arid climatic conditions. Based on geomorphological origin and topographic setting, the desert sediments can be broadly classified into three categories: deposits associated with depressions and valley systems; sediments transported and redistributed by episodic floodwaters and rainfall runoff along the main valley systems; and aeolian sediments formed primarily by wind action. These deposits consist mainly of gravel, sand, gypsum, calcareous material, and varying proportions of silt and clay. The southern sector of the desert contains extensive stony desert pavements composed of coarse sandy and sandstone-derived materials, with sediment thicknesses generally below 10 cm. Much of the natural desert also contains widespread aeolian dune deposits, particularly in the northern and northeastern sectors. In addition, a relatively continuous linear belt of sandy-gravelly desert sediments extends along the eastern margin of the study area adjacent to Basra Governorate. Owing to their loose texture and weak structure, these sediments are readily eroded by wind and water (Figure 2).

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Figure 2. Geographical and physiographic setting of the study area (modified after Hussein, 2022)

Field investigations identified distinct Quaternary sedimentary facies across the study area, reflecting different depositional environments and geomorphological processes:

1. Gypcrete and gypsiferous sediments constitute one of the dominant Quaternary depositional facies in the Al-Muthana Desert. They consist of varying mixtures of sand, silt, clay, and secondary gypsum. Under arid to hyper-arid conditions, these deposits form where winter precipitation combines with limited surface drainage. Rainwater accumulates in low-lying, flat, closed depressions, causing temporary inundation. Subsequent intense evaporation precipitates and concentrates soluble salts, producing extensive saline sabkha environments and evaporite crusts. Sabkha-fill sediments consist mainly of unconsolidated sand, silt, and clay in varying textural proportions, commonly accompanied by gypsum and other evaporitic minerals. These depositional environments reflect hydrological fluctuations, evaporative-concentration processes, and aeolian reworking and constitute important geomorphological and pedological components of the desert. Iraq includes different tectonic zones and physiographic provinces; accordingly, different soil types and erosion processes of varying intensity occur across the country [16].

Table 1. Floristic composition of the study area

Geological substrates and sediment characteristics strongly control plant distribution in the Al-Muthana Desert. The highest species richness was recorded at stations situated on depression-fill clastic deposits (St1, St2, and St8). By contrast, gypsiferous and gypcrete deposits (St3 and St6) were characterized by low species richness and were dominated by halophytic and gypsophytic species because of the restrictive properties of gypsum-rich soils. Likewise, aeolian deposits and dunes (St4 and St5) supported sparse, discontinuous vegetation owing to substrate instability and low water-holding capacity. At St7, coarse sandy-pebbly sediments supported a xerophytic shrub-grass community adapted to arid conditions. These results demonstrate that geological heterogeneity exerts major control over vegetation composition and diversity in the Al-Muthana Desert. Depression-fill deposits provided the most favorable habitats and supported relatively high vegetation cover (Table 2).

Table 3. Diversity indices of vegetation communities across the studied stations

Stations	Species Richness (S)	Shannon Diversity Index (H′)	Simpson Index (1−D)	Evenness (J)	Dominance Index (D)
St1	21	2.710	0.92	0.890	0.074
St2	19	2.760	0.934	0.937	0.066
St3	18	2.790	0.938	0.965	0.062
St4	20	2.840	0.942	0.948	0.058
St5	13	2.420	0.901	0.944	0.099
St6	13	2.370	0.894	0.924	0.106
St7	14	2.450	0.907	0.928	0.093
St8	24	2.880	0.944	0.906	0.056

Table 4. Euclidean distance matrix among vegetation-sampling stations in the study area

Stations	St1	St2	St3	St4	St5	St6	St7	St8
St1	0.000	0.729	1.103	1.129	2.922	3.249	2.506	1.534
St2	0.729	0.000	0.375	0.655	2.967	3.360	2.542	1.554
St3	1.103	0.375	0.000	0.626	3.072	3.488	2.654	1.679
St4	1.129	0.655	0.626	0.000	3.617	4.013	3.192	1.079
St5	2.922	2.967	3.072	3.617	0.000	0.459	0.446	4.417
St6	3.249	3.360	3.488	4.013	0.459	0.000	0.858	4.767
St7	2.506	2.542	2.654	3.192	0.446	0.858	0.000	3.991
St8	1.534	1.554	1.679	1.079	4.417	4.767	3.991	0.000

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Figure 3. Principal component analysis (PCA) ordination of vegetation stations based on species richness and diversity indices

PCA was performed using standardized ecological variables: species richness, the Shannon diversity index (H′), and the Simpson diversity index (1-D). Variables such as evenness and dominance were excluded because they are mathematically related to the included descriptors, thereby avoiding multicollinearity and non-independence. PCA summarized ecological relationships among stations and visualized the contributions of diversity variables to vegetation differentiation across the study area (Table 4, Figure 3).

The PCA ordination revealed differences among vegetation stations based on species richness and the Shannon and Simpson diversity indices. The first principal component (PC1) explained 95.9% of the variance, indicating that most variation among stations arose from differences in these diversity measures. St5, St6, and St7 had positive PC1 scores, suggesting similar richness and diversity characteristics. Conversely, St2, St3, and St4 plotted closer to the Shannon and Simpson vectors than St1 and St5, indicating more similar diversity profiles and stronger associations with these indices. The proximity of St2 and St3 in both the PCA ordination and the Euclidean distance matrix confirms their similarity. St8 was clearly separated from the other stations, indicating a distinct diversity pattern.

St1 occupied an intermediate position, indicating moderate similarity to the other stations. The Euclidean distance matrix showed varying degrees of similarity among stations. The smallest distance occurred between St2 and St3 (0.375), indicating high similarity in richness and diversity indices. High similarity was also observed between St5 and St7 (0.446) and between St5 and St6 (0.459). By contrast, the greatest distances occurred between St8 and St6 (4.767), followed by St8 and St5 (4.417), indicating marked differences in diversity attributes. Overall, St1-St4 and St5-St7 formed two internally more similar groups, whereas St8 remained distinct from the other stations. The PCA results therefore demonstrate substantial spatial variation among vegetation stations based on richness and diversity indices.

Sampling-site distributions corresponded to sedimentary features, woody-plant composition, soil texture, and geomorphology.

1. The depression-associated clastic sediments at Sites 1, 2, and 8 (Salman, Salhubia, and Busaiya, respectively) supported the highest floristic richness and structural heterogeneity among the investigated sites.

The dominance of xerophytic and halophytic representatives of Amaranthaceae, Asteraceae, Lamiaceae, Asparagaceae, Solanaceae, and Fabaceae constituted the principal adaptive flora of the arid Iraqi desert ecosystem. The presence of Amaryllidaceae, Poaceae, Plantaginaceae, and Cucurbitaceae demonstrated additional ecological heterogeneity and microhabitat variability. These communities likely supported higher species diversity than other desert substrates because of greater water availability, sediment stability, and localized nutrient accumulation.

2. The vegetation assemblages at Sites 3 (Shawiya) and 6 (Dudan) were associated with gypsiferous sandy-silty deposits and gypcrete. These habitats had low vegetation cover and low species diversity. In sabkha-type areas, plants generally occurred in isolated patches separated by barren surfaces because of the harsh conditions imposed by saline-gypsiferous substrates. The floristic composition was simple and dominated by stress-tolerant taxa from Asteraceae, Solanaceae, Amaryllidaceae, Plantaginaceae, Rhamnaceae, Tamaricaceae, and Zygophyllaceae. These families tend to be associated with hypersaline, gypsum-rich habitats subject to severe drought because of their specialized physiological and morphological adaptations.

3. The sandy terrain and dune sites at St4 (Tachadeed) and St5 (Anssab) contained mobile sandy substrates and unstable geomorphological conditions that strongly limited vegetation establishment and persistence. Vegetation was sparse and discontinuous, occurring as isolated individuals or clumps on the more stabilized parts of dunes. Although overall density was low, psammophytic species were well represented. The dominant and floristically important families were Asteraceae, Boraginaceae, Plantaginaceae, and Capparaceae. Taxa inhabiting aeolian environments exhibit several adaptations, including extensive root systems, small leaf areas, pubescence, and tolerance of wind erosion and sand burial, which enable survival in these dynamic environments.

4. At Site 7 (Amghar), the sandy-pebbly deposits of the Dibdibba Formation were associated with low floristic diversity and a simplified community composition because of the coarse texture and low water-holding capacity of the substrate. The Dibdibba sediments are well-drained deposits composed of gravelly sand and pebbly material. Vegetation in this habitat was dominated primarily by Asteraceae and Ephedraceae, reflecting their ecological tolerance of dry, nutrient-poor conditions.

The Al-Muthana Desert ecosystem supports a structurally simple but ecologically specialized vegetation assemblage associated with strong edaphic, hydrological, and climatic gradients characteristic of a saline arid landscape. The vegetation shows clear ecological differentiation among habitats, and its distribution is associated with soil salinity, substrate type, moisture availability, and geomorphological diversity. Similar patterns have been reported in Iraqi desert ecosystems and other arid environments of Southwest Asia [20, 21].

The occurrence of ruderal and disturbance-tolerant species, such as Malva parviflora and Capparis spinosa, indicates localized disturbances, including grazing pressure, sediment displacement, and sporadic human activity. Such species frequently occupy ecologically unstable habitats and can indicate habitat disturbance and transitional environmental conditions. The loss of a plant species or group of species within an ecosystem disrupts its functional integrity [22].

Relationships of geology and geomorphology with plant life forms. The structure and relative abundance of plant life forms in the Al-Muthana Desert are closely associated with geomorphological and sedimentological variation across the study area. Field observations indicated that the desert landscape comprises four principal geomorphic units: depression-fill sediments, gypcrete and gypsiferous deposits, aeolian deposits and dunes, and sandy-pebbly sediments. These geological units create distinct soil conditions that affect plant distribution, vegetation composition, and the abundance of life forms at the investigated sites.

Although several sampling sites shared similar broad geological classifications (e.g., St4-St6), differences in salinity, soil texture, and moisture availability resulted in pronounced variation in floristic composition and vegetation density. This finding indicates that plant distribution in the Al-Muthana Desert is determined not only by geological substrate but also by associated environmental factors, including salt accumulation, soil stability, and water availability. Plant families with broad ecological amplitudes contribute substantially to vegetation and soil stability in moderately disturbed desert habitats [23].

Gypsiferous soils and depression-fill sediments are among the most extensive geomorphological units in the study area; species belonging primarily to Amaranthaceae and Asteraceae were associated with soils containing high gypsum concentrations or fine material derived from aeolian transport, runoff, and erosion of the surrounding uplands.

Moreover, aeolian dune systems predominantly supported taxa from Tamaricaceae, Malvaceae, and Zygophyllaceae. Wind activity creates these geomorphological features through continuous erosion, transport, and deposition. The dominance of these families in dune habitats is associated with their ability to withstand drought stress, sand burial, substrate instability, and low nutrient availability. Several species in these groups possess deep roots, small leaves, and other adaptations that enable persistence in shifting sands [24, 25]. Wild plant species face an increasing risk of extinction as climate change raises temperatures and reduces rainfall, thereby intensifying environmental stress and increasing the vulnerability of natural vegetation.

Variation in the abundance of plant life forms in the Al-Muthana Desert demonstrates that life-form patterns are strongly influenced by geomorphological processes.